Partially, Lestat had hoped that Eric was too stupid to think of experimenting. He hoped it'd just been a ridiculous run of luck. This was because, simply, he wouldn't have thought to experiment--not without someone else's suggestion of it--and refused to believe that Eric might in any way be superior.
... but you might get a day.
There must be another catch. Lestat's other eyebrow rose, both forming an odd canopy above eyes that managed, despite everything, not to look surprised.
Just curious.
Drinking Eric's blood could have odd side effects. Eric was older, yes, but he was another kind of vampire. One that was, from what Lestat inferred, weaker in several ways. If Eric wished his blood in return... Lestat was not sure he could offer that. He'd been made to promise not to make any fledgelings (and look how well he'd kept to that promise) due to the strength of his blood. His fledgelings were monsters, stronger than they should be by centuries, thanks to Akasha, and Maharet, and Marius, and, he imagined, Christ.
"Oh?" He gave Eric a slight smile, one that was laced with less nice things underneath. "What's the catch?"